Electromagnetic acoustic transducer (EMAT) for ultrasonic inspection of liquids in containers

ABSTRACT

An ultrasonic inspection technique using a specially designed electromagnetic acoustic transducer (EMAT) launches and receives longitudinal ultrasonic waves into a thin metal wall or thin metal foil seal of a container, causing it to vibrate and launch ultrasonic compressional waves into liquid contained therein. The contents of plastic containers having a metal foil seal forming one wall are easily inspected. The EMAT establishes a magnetic field in the surface of the metal parallel to the surface. Radio frequency (RF) eddy currents are also induced by the EMAT in the surface of the metal. A Lorentz force is generated in the metal surface according the vector product of J, the current density, and H, the magnetic field, and the force generated by the interaction of the perpendicular components of the magnetic field H and the eddy currents J is directed normally to the surface of the metal. This normal force oscillates with the frequency of the induced eddy currents creating ultrasonic compressional waves which propagate normal to the surface of the metal. In such thin-walled metal containers or thin metal foil seals, where the thickness of the metal is much shorter than the ultrasonic wavelength in the metal, the generation and reception process is analogous to the operation of a loudspeaker in air. In this application, the thin metal wall or foil acts as a membrane, with the Lorentz forces generated in the wall or foil causing the metal membrane to vibrate, generating ultrasonic waves in the liquid. Because the thin wall or foil is much more compliant than a thick piece of metal, much larger displacements are generated at the metal-liquid interface than for the thick-wall case, resulting in much larger signal amplitudes than in the thick-wall case.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to the use of ultrasonics toinspect liquids in containers and, in particular, to a method andapparatus for ultrasonically inspecting liquids in containers todetermine the condition of the liquids wherein the ultrasonic waves areproduced by electromagnetic acoustic transducers (EMATs).

The use of ultrasonics to inspect liquids inside containers usingconventional ultrasonic testing methods is well established in the art.In such testing, a piezoelectric (or similar) transducer is coupled tothe wall of the container using some form of coupling media such aliquid. The ultrasonic sound waves then propagate through the wall ofthe container and into the liquid inside the container. The sound wavemay then reflect from a solid object in the liquid, or a liquid-airinterface, or the opposite wall of the container, and be detected by theultrasonic transducer. In other applications, a pitch-catch arrangementof two transducers on opposite sides of the container is used to launchand detect the ultrasound. A wide range of measurements and liquidsusing ultrasonics is possible. Some of the possible measurements includeliquid height, detection and imaging of solid objects in liquids (forexample medical ultrasonic imaging of internal organs), velocitymeasurements of liquid flow, and attenuation measurements to determinethe condition of the liquid.

Nagata et al. (U.S. Pat. No. 4,821,573) discloses a method and apparatusfor ultrasonically inspecting the food contents of a package. Anultrasonic transmitter-receiver system is disposed on at least one sideof the package and the occurrence or degree of degradation of thecontents is determined based upon the output data from the system. Theinvention is stated to be particularly useful to inspect foods,pharmaceutical agents, feedstuffs and so-on, and they may be of anydesired consistency but only if it is freely-flowable, such as ahomogenous solution, a dispersion, a paste, or the like. Before thepackage is subjected to the ultrasonic inspection, it is preferablyshaken so as to disperse the headspace (plenum within the packagingmaterial) into the contents. However, depending upon the types ofcontents, the shaking process may be omitted. This system may bedisposed either in contact with the exterior surface of the package or ashort distance therefrom and may be disposed on the exterior wallsurface of a water tank when the packages are subjected to ultrasonicinspection while immersed in water. Transmitters can be disposed on onlyone side of the package with the ultrasonic receiver on the other side,or the ultrasonic transmitter and receiver can both be disposed on thesame side of the package so that the receiver receives a reflectedultrasonic wave. By measuring differences in sonic velocities between atransmission wave and a reception wave or the sonic velocity of areception wave, the time from transmission to reception, and/or thedegree of attenuation of the ultrasonic energy, the occurrence anddegree of degradation of the contents can be evaluated. The usefulwavelength of the ultrasonic waves disclosed is about 0.5 MHz to about20 MHz. While only a single frequency can be employed at a given time,the accuracy of the evaluation is said to be improved by using a fewselected frequencies for the transmission wave. Various sets of testsresults based on measurements of reflected waves, transmitted waves, andthe like are provided to show how various signals can be related to thecondition of the food product contents.

As indicated in the various references cited on the face of Nagata etal., sonic or ultrasonic assessment of the condition of foodstuffs hasbeen around for some time.

Clark et al. (U.S. Pat. No. 2,277,037) discloses a fruit ripeness testerwhich measures the degree of ripeness of fruit such as melons andpineapples by the measurement and calibration of the vibrationcharacteristics of such objects.

Martner et al. (U.S. Pat. No. 3,357,556) discloses a method andapparatus for testing canned liquid material without removing thematerial from the can, and is particularly suited for the inspection ofbatch-prepared infant formula. The system is used to detect alterationsin viscosity distribution such as by formation of curds or semi-solidbodies or the like, or increases in viscosity as a liquid material ageswhich is referred to as "age-thickening". The cans containing the liquidare rolled along a horizontal path at a preselected constant speed andin the path is disposed a narrow barrier having a height that is smallwith respect to the diameter of the can. By proper adjustment of theheight of the barrier, cans containing a liquid material that issatisfactory will pass over the barrier whereas cans containing spoiledor aged-thickened contents will be arrested by the barrier. The"slushing" or flow pattern of the canned liquid material within the cansis responsible for cans "bouncing" off of the barrier and rollingbackwardly.

Baird (U.S. Pat. No. 3,553,636) discloses a non-contacting ultrasonicinterface viscosity and percent solid detecting device wherein thetransducer is mounted out of contact with the processed liquid. Changesin ultrasonic attenuation characteristics are used as indications ofchanges in viscosity, percent solids, and/or interface level conditionof the liquid contained within a vessel.

Kreula et at. (U.S. Pat. No. 3,913,383) discloses a method and apparatusfor testing the contents of packages containing liquid product. Packagesof interest are sealed and contained liquid food products have physicalproperties that can change as a consequence of deterioration of theproduct. The package to be tested is placed on a movable support whichis subjected to a sudden movement of short duration. A characteristicdependent on the movement of the support is detected, and signals aregenerated in response to the detected characteristics and compared withthe preselected reference signals. In essence, the hydrodynamic behaviorof the contents of the package are used to determine whether or not thecontents have changed or spoiled.

Edwards (U.S. Pat. No. 4,208,915) discloses a method for determiningforeign material in food products using ultrasonic sound. A plurality oftransducers are disposed in a rotatable cylinder having a liquidcouplant. The cylinder has a surrounding flexible wall which iscompressed on top of the surface of the food products. The soundfrequencies are transmitted through the food products and received backby a receiver in the transducers for monitoring any variance in thefrequency which indicates foreign material in the food products. Blacket at. (U.S. Pat. No. 4,384,476) discloses a method and apparatus forultrasonically inspecting foodstuffs in which the fluid is passedthrough a curtain of ultrasonic sound. Reflection or absorption of theultrasonic sound by extraneous materials is detected by ultrasonic soundreceiving means and appropriate indication of such detection is given.The foodstuffs being inspected, however, are inspected as they flow passthe inspection point, and have not yet been packaged.

Jarman et al. (U.S. Pat. No. 5,372,42) discloses an ultrasonicinspection method for determining the seal integrity of the bond linesin sealed containers. These particular packages have a lid bonded to acontainer rim and it is the seal between the lid and the container thatis to be inspected. The container rim is disposed between an ultrasonictransmitter system and an ultrasonic receiver system for inspection.

Wertz et al. (U.S. Pat. No. 5,167,157) discloses an ultrasonic methodand apparatus for inspecting laminated products, particularly todetermine the thickness of the innermost layers of the article. The meanof the measurements from each transducer placed on either side of themultilayered article is calculated to determine these thicknesses. Thearticles themselves are laminated plastic articles.

Hayward et al. (U.S. Pat. Nos. 3,832,885 and 3,802,252). Hayward et al.'885 discloses a method and apparatus for inspecting sealed containers,such as vacuum-packed cans of food, which involves repeatedly energizingelectromagnetic transducer coils mounted in close proximity to thecontainers to cause the enclosures of the containers to vibrate at afrequency that is the function of the internal pressures within thecontainer. Sounds produced by the vibrating enclosures provides a tonalpattern distinctive of the presence or absence of a container with anunsatisfactory internal pressure. Changes in the internal pressure areindicative of leakage or food spoilage or corrosion of the container.Hayward et al. '252 is drawn to an apparatus and method for monitoringthe pressure or vacuum in a sealed container which consists of strikingthe can with a magnetic pulse of force to cause it to vibrate freely andthereby generate an acoustic ping sensed by an electrical pick-updevice. The frequency of the ping is a function of the internal pressurein the container and the frequency spectrum of the signal output of thepick-up device is examined at a discriminator circuit to determine ifthe signal output contains selected frequencies at an energy levelindicative of the desired pressure level. If not, the container isrejected. Measures are taken to render the signal output of the pick-updevice insensitive to both the ambient noise and the large noise pulsegenerated while the can is being subjected to the magnetic pulse offorce and also so that only the purest part of the signal generated inresponse to the acoustic ping is examined by the discriminator circuit.

A liquid or gel couplant is required for conventional ultrasonic testingof liquids in containers, which represents an additional cost. However,conventional ultrasonics using liquid or gel couplants are not practicalat high temperatures. Additionally, in some cases, the surface of acontainer to be inspected may be contaminated with hazardous materialswhich would cause the couplant used in conventional ultrasonic testingto become contaminated requiring it to be treated as hazardous materialfor clean up and disposal. Finally, some metal or partially metalcontainers have an outer layer of non-conductive material such as paint,wrappers, labels, lids, or coating that would prevent the coupling ofultrasound into the container from a conventional ultrasonic transducer.

Electromagnetic acoustic transducers (EMATs) are sensors which arecapable of launching and receiving sound waves in metals without acoupling media or even without contact with the surface of a workpiecebeing inspected. The ultrasound is launched and received in the surfaceof the metal by the interaction of magnet fields and eddy currentsgenerated by the EMAT. EMATs are finding a wide range of nondestructivetesting applications for metals. One previous application of EMATs toliquid measurements is known. In this case an EMAT was used to launchtwo types of plate wave modes in the metallic wall of a vesselcontaining a liquid. One mode was the N=1 Shear Horizontal (SH1) mode.The other mode was the N=O Symmetric (SO) Lamb wave mode. The SH1 modepropagates in the wall of the vessel, producing only shearingdisplacements at the liquid-metal interface inside the vessel. Because aliquid cannot support a shear force, the SH1 mode is not attenuated bythe liquid as it propagates. The SO mode produces substantialdisplacements normal to the surface of the metal at the metal-liquidinterface as it propagates. Because a liquid does support compressionalwaves which are generated by displacements normal to the surface of themetal at the metal-liquid interface, the SO mode is attenuated by theliquid as it propagates. By placing a transmitter EMAT and a receiverEMAT a fixed distance apart vertically on the wall of the vessel, andmeasuring the relative amplitudes of the two modes, an indication of theliquid level between the two transducers can be obtained.

As previously noted, certain types of liquid containers have an outerlayer that prevents inspection of the liquid contents therein usingtraditional ultrasonic techniques. Some containers are provided withouter plastic lids, caps or the like which leave an air gap between theoutside surface of the container and the lid sealing the liquidcontents. Traditional ultrasonic inspection techniques requitingphysical coupling between the transducer and the workpiece beinginspected cannot inspect through such air gaps. Some types of containershave thin metal walls or at least one thin metal foil wall.

It is thus apparent that an improved technique for the ultrasonicinspection of liquids in these types of containers is needed and wouldbe welcomed by the industry.

SUMMARY OF THE INVENTION

The present invention overcomes the problems identified above, and otherproblems, and provides a method and apparatus which can be used toultrasonically inspect liquids in containers having thin metal walls orat least one thin metal foil wall. The invention uses an electromagneticacoustic transducer (EMAT) to generate the ultrasonic pulses. Thecouplants or gels used in conventional ultrasonic techniques are notrequired. This facilitates automated inspection of liquids in suchcontainers, and eliminates measurement errors caused by the use ofcouplant and the necessity to clean the couplant from a container wallafter testing. Naturally, the cost of providing the couplant for testingis also eliminated. Liquids in certain metal or partially metalcontainers at very high temperatures can be inspected because of thenon-contact nature of the transducer.

A specially designed EMAT transducer assembly is disclosed and used tolaunch and receive longitudinal ultrasonic waves into a thin metal wallor a thin metal foil wall of a container, causing it to vibrate andlaunch ultrasonic compressional waves into the liquid contained therein.In contrast to applications of an EMAT to thick-walled inspections, thepresent invention is directed to the ultrasonic inspection/generation oflongitudinal ultrasonic sound waves in very thin walled metal containersor metal parts of containers, and is particularly capable of generatinglongitudinal ultrasonic waves in a metal foil seal wall adhered to theother walls of a nonconductive vessel containing a liquid. As usedherein, the distinction between thick-walled and thin-walled is that athin-wall has a thickness that is much shorter than the ultrasonicwavelength that would be propagated through the metal forming the wall.

In each application, the generation mechanism for creating theultrasonic waves is essentially the same. An EMAT is used to produce andto establish a magnetic field in the surface of the thin metal wallparallel to the surface. Radio frequency (RF) eddy currents are alsoinduced by the EMAT in the surface of the thin metal wall. A Lorentzforce is generated in the metal surface according to the vector productof J, the current density, and H, the magnetic field. If H and J bothlie in the surface of the metal wall, the force generated by theinteraction of the perpendicular components of the magnetic field H andthe eddy currents J is directed normally to the surface of the metalwall. This normal force oscillates with the frequency of the inducededdy currents creating ultrasonic compressional waves which propagatenormal to the surface of the thin metal wall.

In thin-walled containers or foils, where the thickness of the metal ismuch shorter than the ultrasonic wavelength in the metal, the generationand reception process is analogous to the operation of a loudspeaker inair. In this case, the thin metal wall or foil acts as a membrane, withthe Lorentz forces generated in the wall or foil causing the metalmembrane to vibrate, generating ultrasonic waves in the liquid. Becausethe thin wall or foil is much more compliant than a thick piece ofmetal, much larger displacements are generated at the metal-liquidinterface than for a thick-wall case, resulting in much larger signalamplitudes. If a metal foil is adhered to a side of a non-conductivevessel, such as a plastic container, the EMAT can be used to launch andreceive sound waves that propagate through the metal foil wall of thecontainer, through the liquid, and return to the EMAT after reflectionat a liquid-air or liquid-solid (wall) interface.

Accordingly, one aspect of the present invention is drawn to a method ofultrasonically inspecting liquid contents in a container to determinetheir condition, where the container has a thin metal wall forming atleast one wall of the container. An electromagnetic acoustic transducer(EMAT) assembly is provided proximate to the thin metal wall to produceand cause a magnetic field to exist therein. An eddy current coil of theEMAT assembly is energized with an RF toneburst signal of knownamplitude and frequency to generate a Lorentz force in the thin metalwall and cause it to vibrate and launch ultrasonic compressional wavesinto the liquid contents. The ultrasonic compressional waves are allowedto travel through the liquid contents and reflect off an interface. Thereflected ultrasonic compressional waves return to the thin metal walland cause it to vibrate in the presence of the magnetic field producedby the EMAT transducer assembly. The vibrations of the thin metal wallinduce a voltage in the eddy current coil of the EMAT transducerassembly. An amplitude of the induced voltage in the eddy current coilis measured to determine the degree to which the ultrasoniccompressional waves were attenuated during their passage through theliquid contents, the degree of attenuation being an indication of thecondition of the liquid contents. The measured attenuation is thencompared against preestablished values of attenuation representative ofknown conditions of the liquid contents to determine the condition ofthe liquid contents.

Another aspect of the present invention is drawn to a method ofultrasonically inspecting liquid contents in a container to determine aliquid level H therein, the container again having a thin metal wallforming at least one wall of the container. This method employs an EMATassembly above, which is used to launch ultrasonic compressional wavesinto the liquid contents. The compressional waves travel through theliquid contents and reflect off an interface. The reflected ultrasoniccompressional waves return to the thin metal wall and cause it tovibrate in the presence of the magnetic field produced by the EMATtransducer assembly. The vibrations of the thin metal wall induce avoltage in the eddy current coil of the EMAT transducer assembly. Thetime of flight of the ultrasonic compressional waves through the liquidcontents is measured, and using a preestablished value for a velocity ofthe ultrasonic compressional waves within the liquid contents, theliquid level H of the liquid contents is calculated using the measuredtime of flight and the preestablished velocity value.

Other aspects of the present invention are drawn to variousconfigurations of an electromagnetic acoustic (EMAT) transducer assemblyfor ultrasonically inspecting liquid contents in a container todetermine their condition, the container having a thin metal wallforming at least one wall of the container. In a first embodiment, theEMAT transducer assembly comprises magnet means for producing a strongmagnetic field across an air gap between portions of the magnet means,the magnet means causing a magnetic field to exist in the thin metalwall when the EMAT assembly is located proximate thereto. Mild steelyoke means provide a low reluctance magnetic field return path for themagnetic field. Flexible eddy current coil means, placed above andspaced from the magnet means by spacer means, receive an RF toneburstsignal of known amplitude and frequency to generate a Lorentz force inthe thin metal wall causing it to vibrate and launch ultrasoniccompressional waves into the liquid contents. The flexible eddy currentcoil means has an arrangement of conductors such that electricalcurrents flowing through central conductors all flow in the samedirection, while electrical currents in outer conductors flow in anopposite direction with respect to those in the central conductors, tocreate ultrasonic compressional waves with uniform polarity.

Other embodiments of the EMAT transducer assembly for use in the methodof the present invention are also disclosed.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific benefits attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich preferred embodiments of the invention are illustrated anddescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic representation of an EMAT transducer assemblyaccording to the present invention positioned to inspect the liquidcontents of a plastic container with a metal foil seal;

FIG. 2 is a top view of the EMAT transducer assembly of FIG. 1, taken inthe direction of arrows 2--2 of FIG. 1, showing a first embodiment of aflexible printed circuit eddy current coil configuration according tothe present invention;

FIG. 3 is a representation of an actual oscilloscope trace showing thesignals generated and received in a plastic container with a metal foilseal by the EMAT transducer assembly of FIG. 1;

FIGS. 4A and 4B are bottom and side views of a second embodiment of anEMAT transducer assembly according to the present invention;

FIGS. 5A and 5B are bottom and side views of a third embodiment of anEMAT transducer assembly according to the present invention;

FIGS. 6A and 6B are bottom and side views of a fourth embodiment of anEMAT transducer assembly according to the present invention;

FIGS. 7A and 7B are bottom and side views of a fifth embodiment of anEMAT transducer assembly according to the present invention; and

FIG. 8 is a schematic block diagram illustrating an electromagneticacoustic transducer (EMAT) inspection system for ultrasonicallyinspecting the liquid contents of a container according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings generally, wherein like numerals designate thesame or functionally similar elements throughout the several drawings,and to FIG. 1 in particular, there is shown a first embodiment of anEMAT transducer assembly 10 that has been reduced to practice forgenerating and receiving ultrasonic wave signals a plastic container orcup 12 having a metal foil seal 14 and containing a liquid 16. It isunderstood that while the present discussion is in the context of aplastic container or cup 12 having a thin metal foil seal 14, thepresent invention is equally applicable to the ultrasonic inspection ofthin metal wall containers such as pop or beverage cans, soup cans, andthe like. These containers may or may not have an outer layer ofnon-conductive material such as paint, wrappers, labels, lids or similarcoatings. They may be provided with a plastic cap or lid covering thethin metal foil seal which forms one wall of the container. Similarly,the term liquid or liquid contents embraces liquids, or liquidscontaining solids (e.g., soups, etc.) or even semi-solid or gelatinousmaterials; i.e., the term liquid in its broadest form encompassesmaterials which cannot sustain a shear force.

Returning now to FIG. 1, the plastic cup 12 may not be completely full,and an air gap 18 may exist between the liquid 16 and a wall 20 of theplastic cup 12. While the EMAT transducer assembly 10 need not be incontact with the container, for the present invention to work the liquidcontents must be in contact with either the thin metal wall or the thinmetal foil seal 14 for the vibrations thereof to be able to be fullycoupled into the liquid contents 16. If air gaps 18 occur often, it maybe necessary to orient (invert) the container 12 to ensure that theforce of gravity causes the liquid contents 16 to be in contact with thethin metal wall or metal foil seal 14. In this embodiment, a magnetarrangement 22 for generating magnetic fields in the surface of themetal foil seal 14, parallel to the surface of the metal foil seal 14,is formed by a pair of permanent (advantageously made of neodymium-iron)magnets 24 and 26, and a mild steel yoke 28. The mild steel yoke 28serves as a low reluctance magnetic field return path. The permanentmagnets 24 and 26 are arranged so that the north pole N of one permanentmagnet faces the south pole S of the other permanent magnet, with an airgap 30 between them. This arrangement creates strong magnetic fields 32between faces 34, 36 of the two permanent magnets 24, 26 across the airgap 30. A flexible printed circuit eddy current coil 38 (shown in detailin FIG. 2) is placed above the magnet arrangement 22. The eddy currentcoil 38 is separated from the permanent magnets 24, 26 by anon-conductive spacer 40 and a thin layer 42 of copper shieldingmaterial is placed on top of the permanent magnets 24, 26 between themagnets 24, 26 and the eddy current coil 38 to prevent signal generationin the magnets 24, 26.

FIG. 2 shows how the eddy current coil 38 is connected so thatelectrical currents flowing in central conductors 44 of the eddy currentcoil 38 all flow in the same direction, while the electrical currents inouter conductors 46 flow in an opposite direction with respect to thosein the central conductors 44. Electrical leads 48, 50 and 52 areprovided and connected to the eddy current coil 38 to conduct theelectricity to and from the eddy current coil 38. Referring again toFIG. 1, the plastic cup 12 containing the liquid 16 is placed on top ofthe EMAT transducer assembly 10 as shown. The inspection of the liquid16 inside the plastic cup 12 with a metal foil lid seal 14 and anon-conductive plastic cap 54 is illustrated. The eddy current coil 38is electrically connected to a transmitter/receiver unit (not shown)which drives the transducer assembly 10 with a large radio frequency(RF) current tone burst. The RF tone burst induces mirror image eddycurrents in the metal foil seal 14 through the non-conductive plasticcap 54. Fringing fields from the magnet arrangement 22 cause largemagnetic fields 32 to exist in the metal foil seal 14 essentiallyparallel to a surface 56 of the metal foil seal 14 and oriented from onemagnet face 34 to the other magnet face 36. The two permanent magnets24, 26 are spaced apart by a distance (i.e., the air gap 30) thatcreates magnetic fields only in a central region 58 of the metal foilseal 14 that has eddy currents flowing in one direction. The precisedistance depends on the strength of the permanent magnets 24, 26 and thegeometry, and can be determined experimentally. This is done to createan ultrasonic wave with uniform polarity. Those eddy currents which flowperpendicular to the applied magnetic field 32 create a force normal tothe surface 56 of the metal foil seal 14 which launch compressionalwaves 60 in the liquid 16. These ultrasonic waves 60 travel through theliquid 16 and reflect off a liquidair interface 62 at an upper portion64 of the plastic container or cup 12. The ultrasonic waves 60 thentravel back down through the liquid 16 and cause the metal foil seal 14to vibrate. The vibration of the metal foil seal 14 in the presence ofthe applied magnetic field 32 induces a voltage in the eddy current coil38. This voltage is detected and conveyed by the electrical leads 48 and52 to a preamplifier (not shown) and then sent to the pulser/receiverelectronics (also not shown) where it is further amplified and filtered.The detected voltage signal can then be displayed on an oscilloscope(not shown) or captured by a high-speed waveform digitizer (not shown)and processed or displayed by a computer (not shown). By measuring thetime of flight of the received signal, and knowing the velocity of soundfor the liquid 16 in the plastic container or cup 12, a precise measureof the liquid 16 height H in the plastic container or cup 12 can bemade. This method employs the EMAT assembly 10 to launch the ultrasoniccompressional waves 60 into the liquid contents 16. The compressionalwaves 60 travel through the liquid contents 16 and reflect off theliquid-interface 62. The reflected ultrasonic compressional waves 60return to the thin metal wall 14 and cause it to vibrate in the presenceof the magnetic field 32 produced by the EMAT transducer assembly 10.The vibrations of the thin metal wall 14 induce a voltage in the eddycurrent coil 38. A time of flight of the ultrasonic compressional waves60 through the liquid contents 16 is measured by noting when the eddycurrent coil 38 is pulsed, and the time when the ultrasoniccompressional waves 60 return and induce the voltage in the eddy currentcoil 38. Since the liquid contents 16 are known, a calculation of theliquid height H can be calculated using a preestablished value for avelocity of the ultrasonic compressional waves 60 within the liquidcontents 16, and measured time of flight as determined by thetransmission and reception of the ultrasonic compressional waves 60.

By measuring the signal amplitude, information on the condition of theliquid 16; e.g., whether or not it is spoiled, can be determined. Theactual condition measurement involves a comparison of the measuredattenuation against preestablished values of attenuation representativeof known conditions of the liquid contents. The preestablished valueswould be obtained through laboratory tests, etc. and would allowcomparisons against known standards. The comparisons could be done by ahuman operator comparing a displayed value of attenuation (a signalstrength, etc.) against a table value, or preferably it could be doneelectronically using a programmed microprocessor-based system employinga look-up table stored in memory. The latter technique would facilitateautomation of the method and reduce subjective errors introduced byhuman operators processing the results.

A prototype EMAT transducer assembly 10 was prepared and used togenerate and receive ultrasound in a number of metal or partially metalcontainers and plastic containers to which a thin layer of copper foilwas adhered. FIG. 3 is a representation of an actual oscilloscope traceshowing the received signal in a plastic container 12 with a metal foilseal 14, using the embodiment of FIGS. 1 and 2 to both generate andreceive the ultrasonic signal. By measuring the time of arrival of thereflected ultrasound pulse, the level of the liquid could be determined.By measuring the signal's amplitude, spoilage of the edible foodstuffstherein due to bacteria growth could be detected because the spoilagecaused the detected signal to be attenuated to a greater degree thanthat which occurred in non-spoiled cases. In fact, cases of severespoilage actually caused the received signal to disappear. These testswere performed and signals were induced through a cellophane wrapper anda plastic lid approximately 0.030" thick. This type of package precludesthe use of conventional ultrasonics to perform the measurements. Aprototype EMAT transducer assembly 10 was also used to launch andreceive ultrasonic pulses into liquids inside aluminum beverage cans,steel cans, and plastic containers to which a thin metal foil had beenadhered. The EMAT transducer assembly 10 was also used to launch andreceive signals in liquids contained in rectangular cardboard beverageboxes (i.e., juice boxes) having an interior metal foil lining.Accordingly, the method and apparatus of the present invention has awide range of potential uses such as automated liquid level sensing,automated liquid condition assessment, high temperature liquidinspections, inspection of hazardous liquids in containers, liquid flowvelocity measurements, and the imaging of solids inside liquidcontainers.

Other embodiments of EMAT transducer assembly configurations for use inthe present method are described and shown in FIGS. 4-7. Theseembodiments involve variations on the shape of the eddy current coil, aswell as the structure of the magnet arrangements employed to generatethe required magnetic fields. In all of the following embodiments, theEMAT eddy current coil would be located proximate the thin metal wall ormetal foil seal 14.

FIGS. 4A and 4B shows an embodiment which results in a more efficientEMAT transducer assembly than that of FIGS. 1 and 2. As shown in FIGS.4A and 4B EMAT transducer assembly 70 has a circular eddy current coil72 located proximate to a cup-shaped permanent magnet 74 having anaperture in the center to allow it to be combined with a centralcylindrical pole piece 76 and an outer cylindrical pole piece ring 78 tocreate radially oriented fields. A nonmagnetic structural piece 80 maybe provided to the connect the several elements together as an assembly,but the magnetic fields of the permanent magnet 74 may be sufficient tohold it together as a unit. In this, as well as the other embodiments,the non-magnetic structural piece may be made of any non-magneticmaterial; e.g., plastic, ceramic, stainless steel, aluminum, etc. Thefunction of the structural piece is merely as a holder or support;naturally its actual material (so long as it is non-magnetic, even if aconductor) will be selected to withstand the temperature and otherenvironmental factors required by a given application. The embodiment ofFIG. 4A and 4B would be a more efficient EMAT transducer assembly thanthat described above in connection with FIGS. 1 and 2 because more ofthe eddy current coil 72 would be active in generating and receiving theultrasonic sound waves.

FIG. 5A and 5B discloses another embodiment of an EMAT transducerassembly 90 which uses a rectangular (racetrack) eddy current coil 92with a magnet arrangement having a rectangular central pole piece 94 atthe middle of the rectangular eddy current coil 92, and rectangular polepieces 96, 98 at outer edges of the rectangular eddy current coil 92.Again, a non-magnetic structural piece 100 may be provided to provide arugged, stable transducer assembly. This embodiment causes the magneticfields in the thin metal wall surface of a container being inspected tobe oriented in opposite directions from the center pole piece 94 to theouter pole pieces 96, 98, which results in the induced eddy currents andmagnetic fields reversing at the same time, which causes a uniformpolarity Lorentz force and created ultrasonic pulse.

FIGS. 6A and 6B shows another embodiment of an EMAT transducer assembly110 which employs an array of permanent magnets 112 and thin magneticmaterial pole pieces 114 sandwiched between the permanent magnets 112such that alternating polarity magnetic fields are created parallel to asurface of the metal or partially metal container. A meander style eddycurrent coil 116 would then be constructed such that conductors 118thereof would run between the magnetic pole pieces 114, next to thesurface of the container, with the currents and magnetic fieldsreversing direction together, again resulting in a uniform polarityLorentz force and resulting ultrasonic pulse. Non-magnetic structuralpiece 120 may again be provided.

FIGS. 7A and 7B discloses yet still another embodiment of an EMATtransducer assembly, generally referred to as 130, which also employs ameander style eddy current coil 132 located in between two magnetic polepieces 134, 136 and underneath a single permanent magnet 138. Themagnetic field produced would be parallel to the surface of thecontainer being inspected. Non-magnetic structural piece 140 can againbe provided. The meander eddy current coil 132 would be driven by an RFtoneburst with a frequency given by: F=V_(L) /(2D sin(θ)), where F isthe frequency of the toneburst, D is the wire to wire (or conductor)spacing in the meander eddy current coil, V_(L) is the compressionalwave velocity in the liquid, and 0 is the ultrasonic beam angle withrespect to a line normal to the surface. It would then be possible tosweep the beam (i.e., vary the beam angle) by changing the frequency Fof the toneburst. The advantage of this design is that it would allow anoperator to locate solids or other sound wave reflectors within theliquid contents 16.

Finally, FIG. 8 schematically discloses an electromagnetic acoustictransducer (EMAT) inspection system 150 for ultrasonically inspectingthe liquid contents 16 of a container 12. The system 150 is comprised offour main portions: data acquisition and control computer means 160;programmable EMAT electronics 170; remote EMAT electronics 180; and theEMAT transducer assembly 10. The data acquisition and control computermeans 160 is operative to control the settings of various parametersrelated to the EMAT inspection system, and also serves as the primaryoperator interface with the system 150. Included therein would be meansfor digitizing the signals and processing them through known software.Accordingly, data acquisition and control means 160 could comprise atypical personal computer adapted for these purposes. Integral orconnected thereto will typically be means for displaying receivedultrasonic signals 190 from the container 12, as well as data storagemeans 200 for retaining the signals as data for later analysis. Acomputer interface 210 is provided between data acquisition and controlcomputer means 160 and the programmable EMAT electronics 170; a triggersignal is sent via line 220 to the electronics 170, while dataacquisition start signals and RF output signals are sent via lines 230and 240, respectively. Transmitter cable 250 provides the RF toneburstfrom the programmable EMAT electronics 170 to the remote EMATelectronics 180, while preamplified output signal cable 260 provides thepreamplified signals from the EMAT transducer assembly 10 (via theremote EMAT electronics 180) to the programmable EMAT electronics 170.Line 270 interconnects the EMAT transducer assembly 10 with the remoteEMAT electronics 180 to provide the RF toneburst signal to the EMATtransducer assembly 10 and to provide the received signals back to theremote EMAT electronics 180.

The method and apparatus of the present invention has several advantagesover conventional methods for the inspection of liquids in containers.The method and apparatus can be used to inspect liquids in metal orpartially metal containers that have an outer layer of non-conductivematerial such as paint, wrappers, labels, lids, or coating that wouldprevent the coupling of ultrasound into the container from aconventional ultrasonic transducer.

A liquid or gel couplant is required for conventional ultrasonic testingof liquids in containers. However, the couplant can be eliminated byusing the present disclosure since EMATs are employed, which alsofacilitates automating inspection of liquids in containers, facilitatesthe rapid scanning of liquids in containers, and eliminates errors inmeasurements caused by the use of couplant, as well as eliminating thenecessity to clean off the couplant of a container wall after testing.Naturally, the cost of providing the couplant for testing is alsoeliminated.

The method and apparatus of the disclosure allows liquids in metal orpartially metal containers at very high temperatures to be inspectedbecause of the non-contact nature of the transducer. Conventionalultrasonics using liquid or gel couplants are not practical at thesetemperature extremes.

In some cases, the surface of a container to be inspected may becontaminated with hazardous materials which would cause the couplantused in conventional ultrasonic testing to become contaminated resultingin the necessity to treat it as hazardous material for clean up anddisposal. With the use of EMATs, this potential hazard is eliminated.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, those skilled in the art will appreciate that changes maybe made in the form of the invention covered by the following claimswithout departing from such principles. For example, the presentinvention may be applied to new ultrasonic inspection apparatus for theinspection of containers, as well as to the replacement, repair ormodification of existing ultrasonic inspection apparatus. As analternative to the use of permanent magnets, D.C. (Direct Current)operated electromagnets, pulsed electromagnets, or A.C. (AlternatingCurrent) electromagnets could be used. 0f course, in some embodiments ofthe invention, certain features of the invention may sometimes be usedto advantage without a corresponding use of the other features and thusall such variations and embodiments properly fall within the scope ofthe following claims.

I claim:
 1. A method of ultrasonically inspecting liquid contents in acontainer to determine their condition, the container having a thinmetal wall forming at least one wall of the container, comprising thesteps of:providing an electromagnetic acoustic transducer (EMAT)assembly proximate to the thin metal wall to produce and cause amagnetic field to exist therein; energizing an eddy current coil of theEMAT assembly with an RF toneburst signal of known amplitude andfrequency to generate a Lorentz force in the thin metal wall and causeit to vibrate and launch ultrasonic compressional waves into the liquidcontents; allowing the compressional waves to travel through the liquidcontents and reflect off an interface, the reflected ultrasoniccompressional waves returning to the thin metal wall and causing it tovibrate in the presence of the magnetic field produced by the EMATtransducer assembly, the vibrations of the thin metal wall inducing avoltage in the eddy current coil of the EMAT transducer assembly; andmeasuring an amplitude of the induced voltage in the eddy current coilto determine the degree to which the ultrasonic compressional waves wereattenuated during their passage through the liquid contents, the degreeof attenuation being an indication of the condition of the liquidcontents, and comparing the measured attenuation against preestablishedvalues of attenuation representative of known conditions of the liquidcontents to determine the condition of the liquid contents.
 2. Themethod according to claim 1, wherein the liquid contents are ediblefoodstuffs capable of spoilage due to bacteria and the preestablishedvalues of attenuation representative of known conditions of the liquidcontents identify the degree of spoilage of the foodstuffs.
 3. Themethod according to claim 1, wherein the container is a plasticcontainer and the thin metal wall is a thin metal foil seal forming onewall of the plastic container.
 4. The method according to claim 1,wherein the container is provided with a non-conductive plastic lidinterposed between the thin metal wall and the EMAT transducer assembly,the magnetic field caused by the EMAT transducer assembly passingthrough the non-conductive plastic lid to produce and cause the magneticfield in the thin metal wall.
 5. The method according to claim 1,wherein the interface is defined between the liquid contents and one ofeither vapor contents within the container or a wall of the container.6. The method according to claim 1, further comprising the stepsof:providing an EMAT transducer assembly having magnet means forproducing a strong magnetic field across an air gap between portions ofthe magnet means, the magnet means causing a magnetic field to exist inthe thin metal wall when the EMAT assembly is located proximate thereto,mild steel yoke means for providing a low reluctance magnetic fieldreturn path for the magnetic field, and flexible eddy current coilmeans, placed above and spaced from the magnet means by spacer means,for receiving the RF toneburst signal; and arranging conductors of theflexible eddy current coil means such that electrical currents flowingthrough central conductors thereof all flow in a same direction, whileelectrical currents in outer conductors thereof flow in an oppositedirection with respect to those electrical currents flowing in thecentral conductors, to create ultrasonic compressional waves withuniform polarity.
 7. The method according to claim 1, further comprisingthe steps of:providing an EMAT transducer assembly having a circulareddy current coil and a cup-shaped permanent magnet having a central acylindrical pole piece therein, and an outer cylindrical pole piece ringto create a strong, radially oriented magnetic field in the thin metalwall of the container.
 8. The method according to claim 1, furthercomprising the steps of:providing an EMAT transducer assembly having arectangular eddy current coil and a magnet arrangement having arectangular central pole piece at a middle of the rectangular eddycurrent coil and rectangular pole pieces at outer edges of therectangular eddy current coil to cause the magnetic field in the thinmetal wall of the container to be oriented in opposite directions fromthe central pole piece to the outer pole pieces so that induced eddycurrents and the magnetic field reverse directions at the same time toproduce a uniform polarity Lorentz force and created ultrasoniccompressional wave.
 9. The method according to claim 1, furthercomprising the steps of:providing an EMAT transducer assembly having anarray of permanent magnets and thin magnetic material pole piecessandwiched between the permanent magnets, and a meander style eddycurrent coil whose conductors run between the magnetic pole pieces nextto the thin metal wall to create alternating polarity magnetic fieldsparallel to the thin metal wall of the container; which cause currentsin the eddy current coil and the magnetic field to reverse directiontogether and produce a uniform polarity Lorentz force and createdultrasonic compressional wave.
 10. The method according to claim 1,further comprising the steps of:providing an EMAT transducer assemblyhaving a meander style eddy current coil located in between two magneticpole pieces and underneath a single permanent magnet to produce amagnetic field parallel to the thin metal wall of the container; drivingthe meander eddy current coil by an RF toneburst with a frequency givenby: F=V_(L) /(2D sin(θ)), where F is the frequency of the toneburst, Dis a conductor to conductor spacing in the meander eddy current coil,V_(L) is the compressional wave velocity in the liquid contents, and isan ultrasonic beam angle with respect to a line normal to the thin metalwall; and sweeping the beam by changing the frequency F of the toneburstto permit location of solids or other sound wave reflectors within theliquid contents.